This invention relates generally to digital modems, and, more particularly, to techniques for rapid modem carrier acquisition and synchronization. A modem, or modulator-demodulator, is a device widely used in the transmission of digital data signals over a telephone line or other communications link. At a transmitting end of the communications link, the modem modulates a high-frequency carrier signal in accordance with a specific modulation convention. Basically, the data signals to be transmitted are encoded as changes in frequency, amplitude or phase angle of the carrier signal. At the receiving end, another modem demodulates the carrier signal and reproduces the transmitted data.
Since the transmitting and receiving ends of the communications link are usually remote from each other, the receiving-end modem must be accurately synchronized with the incoming signal before it can be properly demodulated. Various synchronization and timing conventions have been developed, but this invention is concerned with modem operation in accordance with transmission conventions of the type established by a proposed United States federal standard, FED-STD-1007. This standard, which is widely followed by modem manufacturers, establishes a signal format for transmission at a rate of 9,600 bits per second, or 9.6 Kbps. The standard also defines a signal preamble format with which the present invention is principally concerned.
The preamble includes an AGC (automatic gain control) and baud synchronization period of 128 bauds, or 53 ms (milliseconds), followed by an equalizer training period of 384 bauds, or 160 ms. The baud synchronization period, as its name implies, allows the receiving-end modem to synchronize itself accurately to the baud rate of the incoming signal. The training period is used to "train" an adaptive equalizer in the receiving-end modem to respond properly to the incoming signals. During the training period, various parameters of the adaptive equalizer are adjusted to optimize its performance in the demodulation of the data-carrying signal that follows the preamble.
The present invention is concerned with two fundamental problems that arise in the operation of modems under a transmission standard of the same type as FED-STD-1007. The first problem relates to use of the standard in what is known as half-duplex mode. In this mode of operation, a single communications link is used for transmission in both directions, but not at the same time. The proposed standard was intended for use in transmitting relatively large amounts of data in one direction. If bi-directional transmission was a requirement, two transmission links would be used, one for each direction. However, the standard is also applicable to the transmission of relatively small amounts of data in alternate directions at high speeds.
One such application is in the transmission of digitized speech data in telephone communications. Use of digital signals for speech transmission facilitates the scrambling of the data for security purposes. Even where security is not a factor, digitized speech signals are more easily compressed, for better utilization of communications links, and more easily stored and switched from line to line in complex communications networks. While digitized speech signals can be conveniently transmitted in a "full-duplex" mode, using a separate communications link for each direction of transmission, a fifty-percent line cost saving is effected if the half-duplex mode can be used.
A practical limitation of half-duplex operation for the transmission of speech is the time that is required to "turn the line around," so that the sending-end modem begins operating as a receiving-end modem, and vice versa. If the time for resynchronizing the receiving-end modem is unduly long, an objectionable delay occurs between the talking and listening phases of a telephone conversation.
In accordance with the standard, signals are transmitted in a quadrature amplitude modulation (QAM) format, wherein both the amplitude and the relative phase angle of the carrier signal are modulated. In the baud synchronization part of the preamble, a sequence of two signal phasors is transmitted in alternation, the two phasors having different amplitudes and phase angles. At this synchronization stage, a typical receiving-end modem would have in operation at least three adaptive control loops, namely a phase-lock loop, an automatic gain control loop, and a baud synchronization control loop. Because of the alternate phase shifts and amplitude shifts of the incoming phasors, the three control loops tend to interact in an unfavorable manner and to slow the acquisition process. These conventional techniques result in unacceptable delays in the aquisition time during switching of the transmission direction in half-duplex mode.
The second major problem with respect to the federal standard is the timely detection of the start of the equalizer training period in the signal preamble. It is important for the receiving-end modem to be able to determine accurately when the baud synchronization interval of the preamble ends and the equalizer training interval begins. The simplest and most economical procedure for training the modem adaptive equalizer is to generate a required reference pattern in synchronism with the received training pattern. However, to do this requires an accurate knowledge of when the training period starts. Failure to obtain correct synchronization between the received training pattern and the locally generated reference pattern either degrades performance or requires a more complex and costly equalizer to make up for the resulting timing uncertainty. The only other alternative is to try several timing hypotheses in order to find the one that best matches the start of the training period. This also entails additional cost and complexity.
It will be appreciated from the foregoing that there has been a need for significant improvement in the area of modems for use with transmission standards of the same type as FED-STD-1007. In particular, what is required is a technique for reducing the baud synchronization time to such a degree that the transmission standard is acceptable for use in the transmission of speech in half-duplex mode. In addition, what is needed is a technique for the accurate detection of the start of the equalizer training period in the signal preamble prior to transmission of data. The present invention satisfies both these needs.